1. Field of the Invention
The present invention relates to a servo routine of a hard disk drive.
2. Background Information
Hard disk drives contain a plurality of magnetic heads that are coupled to rotating disks. The heads write and read information by magnetizing and sensing the magnetic fields of the disk surfaces. Each head is attached to a flexure arm to create a subassembly commonly referred to as a head gimbal assembly (“HGA”). The HGA's are suspended from an actuator arm. The actuator arm has a voice coil motor that can move the heads across the surfaces of the disks. The heads are typically electronically connected to the circuits of the drive by a flex cable.
Information is typically stored in radial tracks that extend across the surface of each disk. Each track is typically divided into a number of segments or sectors. The voice coil motor and actuator arm can move the heads to different tracks of the disks.
FIG. 1 shows a typical track that has a number of fields associated with each sector. A sector may include an automatic gain control (“AGC”) field 1 that is used to adjust the strength of the read signal, a sync field 2 to establish a timing reference for the circuits of the drive, and ID 3 and Gray Code 4 fields to provide sector and track identification, respectively.
Each sector may have also a servo field 5 located adjacent to a data field 6. The servo field 5 contains a plurality of servo bits A, B, C and D that are read and utilized in a servo routine to position the head 7 relative to the track. By way of example, the servo routine may utilize the algorithm of ((A-B)-(C-D)) to create a position error signal (“PES”). The PES is used to create a drive signal for the voice coil motor to position the head on the track.
The servo fields are written during the manufacturing process of the disk drive. The servo is typically written with a precise servo writer. Any vibration in the drive and/or servo writer during the writing of servo may create repeatable run-out(“RRO”)in the servo fields. Repeatable run-out can increase the settle time during a seek routine and ultimately increase access time to retrieve data from the drive.
Some servo routines utilize a repeatable run-out estimator in a feed forward control loop to account for repeatable run-out on the disks. Unfortunately, prior RRO estimates did not account for DC torque disturbances such as flex circuit bias. If a DC torque disturbance is present in the drive, the servo may not converge to zero. It is therefore desirable to provide a servo that does account for the torque disturbances.